1. Field of the Invention
The present invention relates to a display device designed to form optical images by modulating the incident light, and also a projection-type display apparatus realizing a large-screen display by magnifying and projecting images formed by the display device onto a screen.
2. Description of the Prior Art
Because of the light-weight and thin-size of display devices using a liquid crystal (referred to as a liquid crystal display device hereinafter), enthusiastic researches and developments have been directed to this field lately. For instance, a pocket-size TV using a twist nematic (TN) liquid crystal which applies the optical rotatory power of the liquid crystal has been brought on the market and moreover, a projection-type display apparatus utilizing the aforementioned TN panel as a light valve has also been in practical use
The above TN liquid crystal panel requires the incident light to be linearly polarized through a polarizing plate for modulation. However, the polarizing plate absorbs or reflects 50% or more of the incident light, and therefore, the using efficiency of light becomes poor in the liquid crystal display device using the TN liquid crystal panel. The display luminance is disadvantageously low as well.
A proposal to solve the above problem is a liquid crystal display panel using a polymer dispersion liquid crystal in place of the TN liquid crystal. The polymer dispersion liquid crystal panel can modulate the light without using a polarizing plate. The polymer dispersion liquid crystal will be briefly depicted hereinbelow.
The polymer dispersion liquid crystal is roughly separated into two types as follows based on the dispersed state of a liquid crystal and a polymer material. Drops of a liquid crystal are dispersed in a polymer material in one type, that is, the liquid crystal is present in the polymer material in the discontinuous state (which will be represented as PDLC hereinafter, and a liquid crystal panel using PDLC will be denoted as a PD liquid crystal panel). The other type of the polymer dispersion liquid crystal has a network of polymers spread in a liquid crystal layer, most similar to the state of a liquid crystal contained in a sponge. The liquid crystal in the structure of a network is continuous, not appearing in the form of drops (referred to as PNLC hereinafter). The display of images in the above two kinds of liquid crystal panels is achieved by controlling scattering and transmission of light.
PDLC utilizes the fact that the index of refraction is different depending on the direction of orientation of the liquid crystal. Without a voltage impressed, drops of the liquid crystal are orientated in irregular directions. Since the index of refraction of polymers becomes different from that of the liquid crystal at this time, the incident light is scattered. On the other hand, when a voltage is impressed to PDLC, the direction of orientation of all the liquid crystal molecules is rendered uniform. Therefore, if the index of refraction when the liquid crystal is orientated in one direction is preliminarily matched with the index of refraction of polymers, the incident light is allowed to pass through the liquid crystal layer without being scattered.
In contrast, PNLC makes use of the irregularity itself in the orientation of liquid crystal molecules. In the irregularly orientated state, namely, when a voltage is not added to PNLC, the incoming light is scattered. When a voltage is supplied to both PDLC and PNLC so that the liquid crystal is orientated regularly, the liquid crystal layer becomes transparent, so that the light is transmitted.
The above PDLC and PNLC are generally termed as polymer dispersion liquid crystals, and PD liquid crystal panels and PN liquid crystal panels are called as polymer dispersion liquid crystal panels. Resin components in the liquid crystal layer are called as polymers.
The operation of the polymer dispersion liquid crystal panel will be described briefly by an example of the PD liquid crystal panel.
FIGS. 56 and 57 are explanatory diagrams of the operation of the PD liquid crystal panel. A pixel electrode 51 is connected to a thin film transistor (not shown, and designated as TFT hereinafter). A voltage is fed to the pixel electrode 51 by turning ON/OFF of the TFT. Upon receipt of the voltage, the direction of orientation of liquid crystal molecules 382 in the form of drops on the pixel electrode 51 is changed.
As indicated in FIG. 56, while a voltage is not supplied (in the OFF state), drops of liquid crystal 382 are orientated irregularly in direction. In this state, the index of refraction differs between a polymer 381 and the liquid crystal molecules 382, whereby the entering light is scattered. When a voltage is added to the pixel electrode 51, the liquid crystal molecules are orientated uniformly as is clearly shown in FIG. 57. If the index of refraction of the liquid crystal molecules when orientated in one direction is set equal to the index of refraction of the polymer 381 beforehand, the incident light is not scattered, but is projected from an array substrate 11.
U.S. Pat. No. 4,435,047 discloses an example of the above-described polymer dispersion liquid crystal or a similar display device. A nematic liquid crystal is sealed in a capsule held between two electrodes according to the U.S. patent liquid crystal device. The display device scatters the light when an electric field does not act to the liquid crystal layer, and passes the light when an electric field is applied to the liquid crystal layer.
Further, another U.S. Pat No. 4,613,207 describes an example of a projection-type display apparatus which projects images obtained by the above U.S. patent liquid crystal display device after magnifying the same. A reflecting-type or a transmitting-type liquid crystal display device is used as a light valve in the apparatus of U.S. Pat. No. 4,613,207. Images displayed by the light valve are projected onto a screen on an enlarged scale.
In a display device using PDLC (referred to as PDLCD), the light is modulated by switching the same to be scattered or transmitted, thereby to form images. More specifically, when the light is scattered, a black display is obtained. On the other hand, when the light is transmitted, a white display is gained. The display contrast represents the ratio of the transmission light of the white indication (referred to as an ON light hereinafter) and the transmission light of the black indication (referred to as an OFF light). Since the amount of the ON light of PDLCD is considerably large because of the transmissible state of the liquid crystal layer, it is necessary to reduce the amount of the OFF light in order to obtain a large display contrast, and eventually it is required to improve the scattering efficiency of light. Although the scattering efficiency may be improved if the thickness of the liquid crystal layer is increased, it brings about another requirement to raise a voltage to make the liquid crystal layer transmissible. The required voltage is limited to be within .+-.6-.+-.7 up to the standard, partly depending on the driving performance of a source drive IC which outputs image signals to the pixel electrode. It is to be noted here that it is a perfectly scattering state when the light is scattered best, that is, the image display surface of the liquid crystal device shows the same luminance in any direction.
Recently, the number of pixels in the liquid crystal panel has been increased more and more. Even such a liquid crystal panel that includes not smaller than a million pixels has been produced experimentally for use in a superfine display panel. The more the number of pixels is increased, the higher the operating clock of the drive IC becomes. And, the output of the drive IC is necessary to be changed in synchronization with the operating clock. .+-.6V is nearly the upper limit when the operating clock is 20 MHZ in the semiconductor technology at present.
From the above reason, the thickness of the liquid crystal layer should be set so that the liquid crystal layer is transmissible at .+-.6V. However, the scattering efficiency of the liquid crystal layer which becomes transparent at .+-.6V is far from the perfectly scattering state described above.
If the source drive IC is greatly enlarged in size or by the like arrangement, .+-.8V or higher driving voltage may be realized. In this case, however, the size of a chip of the source drive IC is undesirably increased, causing the cost rise of the chip. At the same time, since the amplitude of output signals of a gate drive IC for scanning signal lines should be widened as well, the voltage stress of TFT controlling signals impressed to the pixels is increased, which leads to a shorter life of PDLCD.
As described hereinabove, the conventional PDLCD is considerably difficult to fulfill the high contrast of displays although it achieves high luminance displays. Therefore, high contrast displays naturally cannot be expected in a projection-type display apparatus using the conventional PDLCD as a light valve.